The invention relates to nondestructive inspection of solid materials using ultrasonic waves. Especially, the invention relates to a method for determining a residual between a reference signal and a test signal of an inspected solid material, and to a system for providing such a residual. An image of any defect of the inspected material can be calculated from the residual.
Different methods for non-destructive inspection of fluid and solid materials using ultrasound techniques have been developed. Such methods include transmitting an ultrasonic signal into the material and measuring a resulting ultrasonic signal that has travelled through the material at a measuring point, which resulting ultrasonic signal for example has been reflected inside the material before arriving at the measuring point. The analyzing of the resulting ultrasonic signal provides an image of the interior of the material.
Ultrasonic inspection of fluids may start from analysis of pressure. The arrival times of pressure wave echoes give an indication of a distance to a pressure altering structure in the fluid.
The problem of inspecting solids is generally more difficult since solid materials may contain and transfer stress from both compression and shear. Solid materials therefore transfer energy in the form of shear waves as well as compression waves. Ultrasonic inspection of solids is often based on an analysis of a strain field in the material, which strain field may corresponds to compression and shear in the material. The propagation speed of shearing waves is different from the speed of propagation of compression waves, and the measurements and the analysis of the measured signal need to be performed in a more sophisticated manner than for fluids.
One common method uses a short ultrasonic pulse and analysis of the response. One known way of analyzing the resulting measured ultrasonic signal from an inspected part of a construction is to compare the measured signal to a reference signal obtained from a flawless part. Such a reference signal may also be provided from a FEM analysis (Finite Element Method) of the part. The comparison provides a residual signal, which is subsequently analyzed.
Mathematical calculations of how ultrasonic signals from small sources in such a part would produce measurable signals at the measuring position is used to determine an indication of the disturbances in the inspected part that may have produced the residual signal.
Thus, the calculations of a forward, or direct, problem of how small sources produce ultrasonic signals propagating through a material is used as a basis for solving the adjoint, or inverse, problem of what sources, i.e. defects, have produced the residual signal.
U.S. Pat. No. 7,654,142 describes a method for obtaining an image of an inspected part. In this method, a reference part is used, which reference part is a flawless part. A first ultrasonic measurement is performed on the reference part, and a second ultrasonic scan is performed on the inspected part. The measuring probe is positioned in the same relation to the reference part as the inspected part during the measurements, at the same height above a corresponding plane to be inspected. A subtraction is performed between the measurements of the inspected part and the reference part, and the topological energy at each position in the part is determined.
The method of U.S. Pat. No. 7,654,142 determines a “cost function” that correlates data obtained from the reference part and data obtained from measuring the inspected part. In this way an indication of the modifications, or defects, in the inspected part is obtained.
In more detail, the measuring probe includes a number of aligned transducers. The transducers transmit an ultrasonic test signal, one transducer at a time, while the other transducers receives. A matrix of all the received test signals are created, which received test signals are compared to corresponding reference test signals from the reference part. The frequency used for the ultrasonic signals is not indicated, but each measuring results in measurements from a plane of the inspected part.
The method of U.S. Pat. No. 7,654,142 uses the topological energy for providing an image of the inspected part. U.S. Pat. No. 7,654,142 aims at simplifying a previous method described in the article “Flaw imaging with ultrasound: the time domain topological gradient method” by N. Dominguez et al (A1, see the reference list at the end of the description). Both methods are performed in the time-domain, but U.S. Pat. No. 7,654,142 determines the topological energy instead of the gradient for each position of the inspected part. In more detail, the field values of the reference part is subtracted from the measured values of the inspected part, thereafter the subtracted residual signal is subjected to a time reversal by inverting the time scale. This time reversal is described in more detail in the article “Flaw imaging . . . ” and in a further article “Time domain topological gradient and time reversal analogy: an inverse method for ultrasonic target detection” (A2, see the reference list).
A problem for using the methods described in U.S. Pat. No. 7,654,142 and the articles A1 and A2 are to obtain an accurate measurement, i.e. how to avoid disturbances to the ultrasonic test signal when applying the ultrasonic test signal to the inspected part and to avoid disturbances when measuring the resulting signal. The process suggested in article A2 is to use water as a transfer medium to transfer the ultrasonic test signal from the transducer into the inspected part for example, as referred to in the article A2 by immersion of the inspected part in water.
A known alternative to immerse the inspected part in water is to direct a beam of water onto the inspected part and use the water beam as a means for transferring the ultrasonic signal.
A disadvantage of using water is that immersing parts in water baths or directing beams of water onto inspected parts makes the handling of parts for inspection complicated, especially for larger parts and structures.
An alternative to water immersion that may be used is attaching the transmitting and measuring probes permanently to the surface of the inspected part. Such attachment may be done on a flawless part during manufacturing and subsequently used for regular inspections. In this way, the distortion induced from the glue layer will be the same and the measurement signal obtained during an inspection can be compared to an original test signal obtained during manufacturing so that the distortion from the glue layer will not influence the difference between the original reference signal and the subsequent test signal. However, for many parts and constructions it may not be suitable to leave measuring probes attached during use, and also the glue layer may be affected during use of such parts and constructions.
Thus, there is a need for facilitating the measuring process, still providing accurate measurement signals, in order to determine a reliable residual signal.